Internal combustion engines, including compression ignition engines and spark ignition engines regularly undergo redesign efforts to improve efficiency and enhance fuel economy. Compression-ignition engines and direct-injection spark-ignition engines are gaining in popularity due in part to improved fuel economy, which may exceed 20% improvement compared to a similarly-sized, conventional spark-ignition engine. Compression-ignition engines and direct-injection spark-ignition engines operate with excess air in the combustion process, which is also referred to as operating lean of stoichiometry. An engine that operates lean of stoichiometry can do so without a throttle valve in the air intake manifold. Stoichiometry is an air/fuel ratio at which there is a sufficient amount of oxygen from the air mixed with the fuel to completely oxidize the fuel during combustion. When air can freely flow into the cylinders on an intake stroke of a combustion cycle, less pumping energy is required, leading to a fuel economy benefit. Engines that operate lean of stoichiometry can be classified as heavy-duty diesel, light-duty diesel, and direct-injection gasoline engines. Heavy-duty diesel engines are distinguished from light-duty diesel engines by their application and method for emissions certification. A heavy-duty engine is used in a high-load application, and is typically certified for use using an engine dynamometer, whereas a light-duty engine is used in a passenger vehicle or light truck, and is certified for use on a vehicle dynamometer.
An internal combustion engine is typically configured with sensors that are operable to measure engine performance and an operator's requirement for power, and output devices that are operable to control engine performance. The sensors include, for example, an engine speed sensor, a torque sensor, a pedal position sensor, and a mass air flow sensor. The design and implementation of engine sensors is known to one skilled in the art.
Acceptance of compression-ignition engines and direct-injection spark-ignition engines has been limited due to the inability to comply with increasingly strict emissions regulations. Emissions regulations that are implemented in countries throughout the world include standards for allowable levels of exhaust gas constituents that are output as a result of the combustion process. The primary regulated exhaust gas constituents include hydrocarbons (HC), carbon monoxide (CO), nitrides of oxygen (NOx), and particulate matter (PM). Engine manufacturers meet various emissions regulations by designing engines, engine control systems and exhaust aftertreatment devices to reduce NOx to nitrogen (N2 ) and oxygen (O2), and to oxidize HC, CO, and carbon and organic portions of the PM to water (H2O) and carbon dioxide (CO2). When compression-ignition engines and direct-injection spark ignition engines operate with a fuel charge that is at an air/fuel ratio that is lean of stoichiometry, the result is low engine-out emissions of CO and HC. However, lean operation also typically results in higher levels of engine-out emissions of NOx and PM.
Engine system developers have sought to reduce NOx and PM emissions of compression-ignition engines and direct-injection spark ignition engines using several different aftertreatment devices and control schemes. The aftertreatment devices include, for example, oxidation catalysts, lean NOx catalysts, NOx adsorber catalysts, diesel particulate traps, oxidation and three-way catalysts, and selective catalytic reduction catalysts. The aftertreatment devices are placed in an exhaust gas feedstream and are used in conjunction with engine management control schemes and added hardware to reduce tailpipe emissions below regulated levels.
As described in U.S. Pat. No. 6,904,752, the disclosure of which is incorporated herein by reference, a NOx adsorber catalyst is an aftertreatment device that typically comprises a ceramic or metal substrate having a washcoat containing noble metals that is able to purify exhaust emissions at elevated temperatures. The washcoat typically contains barium and other alkali metals that adsorb and store NOx while the engine is operating with excess oxygen. The NOx adsorbed by a NOx adsorber catalyst must be periodically desorbed from the catalyst by reduction. If this does not occur, the catalyst eventually becomes saturated, leading to breakthrough of NOx emissions.
Desorption and catalysis of the NOx requires an exhaust gas feedstream that is rich of stoichiometry, preferably with catalyst bed temperatures above 200° C. The temperature of the exhaust gas feedstream also affects the amount of time that is required to reduce NOx adsorbed by the NOx adsorber catalyst. Currently available NOx adsorber catalysts perform optimally when the temperature of the exhaust gas feedstream is in the range of 350-450° C. This exhaust gas temperature range is difficult to achieve with a compression-ignition engine or direct-injection spark ignition engine that is operated under low-speed, light load driving conditions.
Regeneration of the NOx trap can also be achieved through the use of reformate, hydrogen-enriched fuels that can be produced from a variety of sources, including gasoline, diesel, and other liquid or gaseous fuels. On-board reformers for producing hydrogen-enriched reformate fuels are described in, for example, U.S. Pat. Nos. 6,655,130 and 6,832,473 and U.S. Patent Appl. Publ. Nos. 2004/0146458 and 2005/0022450, the disclosures of which are incorporated herein by reference.
Reduction of NOx in the NOx adsorber catalyst comprises having the engine management system change the fuel charge from a lean air/fuel ratio to a rich air/fuel ratio for a predetermined amount of time. When the rich exhaust gas enters the NOx adsorber catalyst, the stored NOx is desorbed from the washcoat and reacts with exhaust gases including CO, hydrogen (H2) and HC in the presence of the noble metals to form water (H2O), carbon dioxide (CO2), and nitrogen (N2). The reduction cycle typically must occur regularly during operation of the engine. The engine management system resumes normal engine operation after reduction is complete. The prior art uses the engine management system to switch the fuel charge from a lean air/fuel ratio to a rich air/fuel ratio by reducing overall air intake or adding fuel during combustion. The reduction of air intake to the combustion cycle is accomplished by a combination of throttling, reduction in boost from a turbocharger, and increase in EGR. These methods adversely affect fuel economy, and potentially also affect engine performance.
The performance of a NOx adsorber catalyst is negatively affected by the presence of sulfur in fuel. Sulfur burns in the combustion process to form sulfates (SO2 and SO3). The NOx adsorber catalyst preferably selects and adsorbs sulfates over NOx. The sulfates are not released and reduced during periodic rich air/fuel ratio operation as readily as NOx is released. As a result, adsorbed sulfates reduce the capacity of the NOx adsorber to adsorb NOx.
Desulfation of the NOx adsorber catalyst requires a periodic excursion of the exhaust gas to high temperatures (catalyst bed temperatures of 650° C.) at a rich air/fuel ratio for an extended period of time, typically requiring minutes of operation. Desulfation must occur periodically over the life of the engine, typically every 3,000 to 10,000 miles or an equivalent number of hours of engine operation, depending on the level of sulfur in the fuel, fuel consumption of the engine, and the NOx storage capacity of the NOx adsorber catalyst.
The prior art also discloses the reduction of NOx emissions using a selective catalytic reduction (SCR) catalyst, an aftertreatment device that includes a catalyst and a system that is operable to inject material such as ammonia (NH3) into the exhaust gas feedstream ahead of the catalyst to reduce the NOx adsorbed by the catalyst. The SCR catalyst, which includes a substrate and a washcoat containing noble metals, is capable of promoting the reduction of NOx by NH3 or urea, which undergoes decomposition in the exhaust to produce NH3.
NH3 or urea selectively combine with NOx to form N2 and H2O in the presence of the SCR catalyst. The NH3 source must be periodically replenished, and the injection of NH3 into the SCR catalyst requires precise control. Overinjection may cause a release of NH3 into the atmosphere, while underinjection may result in inadequate emissions reduction.